[KYUUBI #7118] docs(client): Add comprehensive docs for Engine Pool feature

Adds documentation for the Engine Pool feature to the `Share Level Of Kyuubi Engines` page. The separate and now-redundant `engine_pool.rst` file has been deleted.

### Why are the changes needed?

The `Share Level Of Kyuubi Engines` documentation previously lacked a section on the Engine Pool feature.

### How was this patch tested?

### Was this patch authored or co-authored using generative AI tooling?
Yes. Generative AI tools were used in several stages of preparing this contribution:

- The initial draft of the documentation was written with the assistance of Claude.
- Revisions and suggestions were provided by Gemini.
- Commit messages and pull request descriptions were refined for clarity using Gemini.

Generated-by: Claude 4, Gemini 2.5 pro

Closes #7118 from z1131392774/docs/engine-pool-feature.

Closes #7118

61892a22c [z1131392774] restore unnecessary style changes
02b4c6729 [z1131392774] ci: Re-trigger CI checks
60154e00a [z1131392774] fix with spotless
9f0700774 [z1131392774] docs: remove redundant engine_pool.md
bf586f700 [z1131392774] update with SERVER_LOCAL and Engine Pool
4d34a5bfd [z1131392774] fix format with spotless
9ed2e8a61 [z1131392774] docs(client): Add comprehensive docs for Engine Pool feature

Lead-authored-by: z1131392774 <z1131392774@gmail.com>
Co-authored-by: z1131392774 <156416015+z1131392774@users.noreply.github.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>

docs/client/advanced/features/engine_pool.rst

@@ -1,18 +0,0 @@
-.. Licensed to the Apache Software Foundation (ASF) under one or more
-   contributor license agreements.  See the NOTICE file distributed with
-   this work for additional information regarding copyright ownership.
-   The ASF licenses this file to You under the Apache License, Version 2.0
-   (the "License"); you may not use this file except in compliance with
-   the License.  You may obtain a copy of the License at
-
-..    http://www.apache.org/licenses/LICENSE-2.0
-
-.. Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License.
-
-Enabling Kyuubi Engine Pool
-===========================
-